1. Field of the Invention
The present invention relates to a Coriolis type mass flowmeter for detecting a phase difference in a U-shaped pipe vibration, caused by mass fluid in up-stream and down-stream in the pipe by Coriolis force, and measuring flow.
2. Description of the Related Art
FIG. 1 shows the operation principle of a Coriolis type mass flowmeter.
Reference numeral 1 denotes a U-shaped pipe through which fluid to be measured flows. A permanent magnet 2 is fixed to its front end middle portion and both ends of the U-shaped pipe 1 are fixed to a base 3. Numeral 4 represents electromagnetic drive coils 4 installed in such a way that they sandwich the U-shaped pipe 1 between them and numeral 5 denotes a frame for supporting the electromagnetic drive coils. The support frame 5 is firmly fixed to the base 3. The U-shaped pipe 1 is set to vibrate, while taking the base 3 as its cardinal point, (as with the tuning fork), to lose less vibration energy.
Numerals 11 and 12 denote electromagnetic pickups (or vibration detecting sensors) for detecting displacements of both legs of the U-shaped pipe 1. When the U-shaped pipe 1 is driven or excited at its inherent vibration value (sin .omega.t) by electromagnetic force acting between the drive coils 4 and the permanent magnet 2 which is in opposite to the coils 4 and which is fixed to the U-shaped pipe 1, Coriolis force is generated in fluid flowing through the U-shaped pipe 1.
FIG. 2 shows how the U-shaped pipe 1 is vibrated.
The Coriolis force is proportional to the mass and the velocity of fluid flowing through the U-shaped pipe 1, and its direction is the same as that of a vector product between the moving direction of fluid and the angular velocity at which the U-shaped pipe 1 is excited. Further, the direction of fluid at the U-shaped pipe 1 becomes in reverse between inlet and outlet for fluid. Therefore, torsion torque is caused because of Coriolis force in both legs of the U-shaped pipe 1. This torque changes at the same frequency as the excitation frequency and its amplitude width is proportional to the mass flow rate of fluid. FIG. 3 shows a vibration mode of the U-shaped pipe 1 generated by this torsion torque.
When the amplitude of this torsion vibration torque is detected by the vibration detecting sensors 11 and 12, the mass flow rate of fluid can be found, but, practically, the vibration of the U-shaped pipe 1 becomes in such vibrations that excited vibrations caused by the electromagnetic drive coils are superposed by torsion vibrations caused by Coriolis force, and vibration waveform on the upstream side is expressed by sin (.omega.t-.alpha.) and that on the downstream side by sin (.omega.t+.alpha.). Therefore, signals e1 and e2, detected by the vibration detecting sensors 11 and 12, are expressed as waveforms having a phase difference (.DELTA.t), as shown in FIG. 4. This phase difference becomes different, depending upon the pipe used and its excitation frequency. Providing that the U-shaped pipe 1 is used and that its resonance frequency is 80 Hz, for example, a time difference of about 120 .mu.S is caused in the maximum mass flow rate. In the case of conventional mass flowmeters, sensitivity can be increased up to 1/20 of the maximum range, and it is needed in this case that the indicated value of flow rate is guaranteed to an accuracy of 1%. The time difference is 120 .mu.S at the maximum mass flow rate and range and it becomes 6 .mu.S at a 1/20th range, and the accuracy is 1%. Therefore, a time measuring accuracy of 60 .mu.S is needed.
Various methods are used to measure this phase. The simplest method is to count reference clocks in a time difference gate. (See FIG. 5.)
Pickup signals 20 and 21 on up- and down-stream sides are amplified by amplifiers 22 then two-valued by comparators 23, exclusive "OR" of these two-valued signals is calculated by an exclusive "OR" circuit 24, a gate pulse 25, having a pulse width which corresponds to a time difference in the up- and down-stream pickup signals, is thus obtained, and the phase is then measured by counting the number of reference clocks in a gate. The frequency of the reference clock needs to be higher than about 20 MHz.
On the other hand, FIG. 6 shows a drive circuit for the U-shaped pipe, and FIG. 7 shows signal waveforms at respective sections of the circuit.
Output signals from vibration detecting sensors 11 and 12 are passed through a low-pass filter (LPF) 50 to remove high frequency noise. They are then turned to low impedances by a buffer 51 and amplified by an amplifier 52. Further, they are rectified by a full-wave rectifier circuit 53 and smoothed by a smoothing circuit 54 to detect a vibration amplitude level.
This level is compared with a driven amplitude reference level of a reference voltage generator circuit 63 by a comparator 57 and the difference thus obtained is amplified by a differential amplifier 58. This difference voltage (C) is used as control voltage for a multiplier 59. The input of the multiplier 59 is multiplied by signals A*sin wt which have been obtained by filtering and vibration-amplifying signals from the vibration detecting sensors 11 and 12. A signal A*C*sin wt is thus obtained and current is caused to flow in a drive coil 61 through an exciter circuit 60 thereby to control the vibration amplitude of the U-shaped pipe 1. The vibration detecting sensors 11 and 12, LPF 50, buffer 51, amplifier 52, full-wave rectifier circuit 53, comparator 57, difference amplifier 58, exciter circuit 60 and drive coil 61 form a negative feedback control circuit, as described above, and if no vibration is added to the U-shaped pipe from outside, the pipe is driven by a drive waveform of sin wt.
When pump vibration (caused when no fluid flows) and fluid vibration are large, however, the output of the smoothing circuit 54 is expressed by a changing waveform, as shown by signal 72 in FIG. 7, and when this change is amplified by the differential amplifier 58, excitation waveform is not a sine wave but it has many distortions, as seen in signal 79, shown in FIG. 7. When excitation is created by this distorted excitation waveform, the quality of vibration is degraded and the changing at zero point increases.
In order to reduce the distortion of excitation waveform, it is supposed that the time constant of the smoothing circuit 54 is increased. When so arranged, the output of the smoothing circuit 54 becomes difficult to change, and when frequencies of pump and fluid vibrations are low enough, the U-shaped pipe can be vibrated by a less distorted sine wave while reducing the changing at zero point. When their frequencies are high, however, the vibration amplitude of the pipe changes and, in spite of a difference relative to the instruction value, the responsibility of the pipe decreases because the time constant of the smoothing circuit is large. In short, the pipe cannot be controlled enough so as to remove the difference.